This paper presents a proof of concept of a differential sensor based on the phase-difference of two injection-locked MEMS resonators, strongly coupled through their actuation voltages by a digital mixer. For the first time the feasibility of a fully monolithically co-integrated CMOS-MEMS differential resonant sensor, exploiting the capabilities of the injection-locked synchronization is proved. The principle of the system is first presented, from which optimal design guidelines are derived. The design of the different blocks of the system is then addressed. Our experimental results demonstrate the sensitivity enhancement of the proposed solution, as predicted by theory, and partial thermal drift rejection in a 70 °C range. The simulated and experimental results highlight the critical points of the system design, on which the emphasis of this article is placed.